Garages and Carports - Garage door opener question

I went out of my front door this morning on my way to work and I saw that the garage door was only half closed and that the belt came off the sprockets. It happened during the night when one of my kids came home.
I worked on it after I work with not much luck it will work going up or down 1 or 2 times then the belt jumps off again.
The door goes up and down by itself smoothly, the trolley runs on the bar with no issue's. Does anybody have any ideas what could be wrong?
It's a Chamberlain Whisper drive model 8100 built in or around 1994 or 1995.
Thanks

is it coming off the front or back pulley?

It comes off the back sprocket/gear above the motor. 7 or 8 months ago I replaced the inner plastic gears as they were worn down to nothing. It has worked fine since yesterday.

I disconnected the door from the the trolley and it moves back and forth fine. The only thing is that the belt travels up and down the sprocket over the motor. I'm not sure why it's doing that. It did not climb enough to jump off the sprocket with the door disconnected but apparently it will when the door is attached.

there is a black cap that screws on over top of that drive pulley, did u put that back on, You changed the plastic gear, I would change the whole gear assembly because the bushings just under the drive pulley wear out and the shaft will tip forward loosening the belt and tilting the drive gear forward causing it to derail. Or maybe the belt is too loose

For right now I put the plastic cap on, I never put in back on when I replaced the gears and of course I could not find it so I had to buy another one. So it works for now. It probably worked for six months without the cap then one day it didn't. So I will probably change the whole gear assembly like you suggested as the bushings are likely worn. I was aiming for 1-1/4" or so of spring after tensioning the nut.
Thanks for the help and the replies, I appreciate it.

Take a look at the sprocket on top of the motor unit. If the bearing wears it will let the shaft lean forward and the belt will pop off.